This invention relates to warming trays, hot plates, ranges and other cooking appliances, and more particularly to a cool-touch safety covering for the heated cooking surfaces thereof.
The operating temperatures of home cooking appliances range from about 100.degree. C or so for warming trays to about 600.degree. C for electric ranges at high heat. At these temperatures, the prior cooking surfaces can, as is well known, do cause serious burns to fingers, hands or other surfaces that happen to come in contact with them.
As explained in detail in my copending application Ser. No. 407,532 filed Oct. 18, 1973, for a "Thermesthesiometer," now U.S. Pat. No. 3,878,728 issued April 22, 1975 the contact temperature, T.sub.c, at the interface between the skin of a finger, having a normal skin temperature, T.sub.p, and the surface of a body heated to a higher temperature, T.sub.h, can be expressed as ##EQU1## WHERE .lambda..sub.H AND .lambda..sub.P ARE THE THERMAL INERTIAS OF THE HEATED BODY AND THE FINGER, RESPECTIVELY. Now, for a brief contact time in the order of a few seconds, the contact temperature should not, as explained in my application, exceed about 60.degree. C, if thermal injury and pain are to be avoided.
Expression (1) can be rewritten as ##EQU2## Using, then, the values of 60.degree. C for T.sub.c, 33.degree. for T.sub.P and 0.0014 cal.sup.2 /sec-cm.sup.4 -deg C.sup.2 for .lambda..sub.p, expression (2) reduces to EQU .lambda..sub.h = (T.sub.h - 60).sup..sup.-2 cal.sup.2 /sec-cm.sup.4 -deg C.sup.2 ( 3)
which expresses the maximum value of the thermal inertia of the heated surface, as a function of its temperature, for a maximum allowable skin contact temperature of 60.degree. C. For thermal safety, then, the thermal inertias of cooking or other hot surfaces should be decreased for increased operating temperature, in accordance with expression (3).
Materials of low thermal inertias, however, tend to be more like thermal insulators than thermal conductors. This is because thermal inertia is by definition the product of three terms--thermal conductivity, specific heat, and density--of which thermal conductivity is of greatest influence. Wood, plastic and other low thermal inertia materials are generally poor heat conductors, while metals and other high thermal inertia materials are good heat conductors.
Since a cooking surface must rapidly absorb heat from a heating element and transfer it to a cooking utensil, the prior surfaces have been constructed exclusively from high-conductivity materials, with attendant high thermal inertias and extremely high burn hazards.